The present invention relates to improvements in or relating to a neutron-protection thermal insulating material capable of moderating and absorbing neutrons.
Usually there is a certain possibility that neutrons will leak from a nuclear reactor. The leaking neutrons give rise to harmful radioactivity, which is highly damaging to the human body and equipment. For this reason, the nuclear reactor is surrounded with a heavy concrete containing boron. Such a boron-containing heavy concrete is so heavy that it exercises a great shielding effect on gamma rays. In addition, this concrete contains boron has a great neutron absorption cross section so that it produces excellent neutron absorption. Due to its safe service temperature ranging from 70.degree. to 90.degree. C., however, this concrete cannot be applied directly to the circumference of a reactor vessel maintained at about 280.degree. C. Consequently, a special measure is taken for thermally insulating the heavy concrete shielding material from the heat of the reactor vessel by interposing a thermal insulating material therebetween. As the thermal insulating material, use has heretofore been made of a radiant thermal insulating material comprising the combination of a metal plate and a metal foil. Sine such a radiant thermal insulating material does not contribute to moderating and absorbing neutrons, the neutrons leaking through the reactor can rise in the space between the reactor vessel and the heavy concrete shielding material occupied by the radiant thermal insulating material in scattering fashion to raise the upwardly directed neutron level. This results in a large quantity of neutrons leaking through the upper shielding material.
Accordingly, it has been proposed to use a shielding material capable of thermally insulating the aforesaid heavy concrete from the reactor vessel and also capable of moderating and absorbing neutrons. One such shielding material is composed of asbestos material containing a boron compund which is formed into a plate. But this material has a fatal defect in that, when its neutron-shielding capability is enhanced by increasing its density, it correspondingly suffers an increase in thermal conductivity.